Computer systems have attained widespread use for providing personal computing capability to many segments of today's modern society. Computer systems can usually be defined as desktop, floor standing or portable microcomputer that consists of a system unit having a system processor(s) and associated volatile and non-volatile memory, a display monitor, a keyboard, one or more diskette drives, a fixed disk storage unit, and optional printers. One of the distinguishing characteristics of these systems is the use of a motherboard or system planar board to electrically connect these components together. These systems are designed primarily to give independent computing capability to users and are available for purchase by individuals and businesses.
It is well known that the components of a computer housed within an enclosure and supported from the chassis or connected with the planar board are capable of emitting electromagnetic radiation at various frequencies and that standards have been established for limiting the emissions of such energy to various limits depending on the environment of use of the computer. Further, it is known that higher speed processors and circuits associated with such processors typically emit radiation of differing frequencies and amounts than the earlier designed slower processors. Heretofore, shielding has been found to be an effective means for limiting radiation. Shielding has been accomplished in various ways including providing metal enclosures, and coating or lining the enclosures and covers provided. In achieving shielding in these ways and attenuating radiation, it has been noted that openings provided in such disclosures and covers can present particularly difficult problems in shielding. Further it has been noted that the possible effects of electromagnetic interference can be interruption of or interference with other electronic devices as well as input/output signals necessarily exchanged with the operating components of the microcomputer.
As the development of computer technology has advanced, computers have been established to incorporate increasingly higher data handling speeds in processors, data storage devices and memory. Such higher speeds bring with them greater emissions of electromagnetic radiation, increasing the demand placed on enclosures for the attenuation of such emissions. Other terms used to refer to such requirements have been electromagnetic compatibility and suppression of electromagnetic interference. In efforts to bring emissions within the required standards while attaining the desired high speeds, designs have moved toward a completely shielded enclosure with minimal gaps and openings.
In addition, under Federal Communication Commission (FCC) and European laws, before computer hardware can be offered for sale, it must first meet specific Electromagnetic Compatibility (EMC) requirements. With increasing CPU speeds, each new system or upgrade needs to be tested for compliance with the various regulations. A computer system that has passed such requirements in the past, will need to be re-tested when it receives an upgraded faster CPU since higher internal frequencies will cause different cavities within the box to resonate and emit radiation. Such new radiation may also be introduced because faster CPUs require different support circuits thereby creating a substantially new combination of electromagnetic radiators within a differently structured and configured CPU enclosure. New components will change the geometry of the radiating structures and spaces between components within the CPU enclosure. Practically all computer enclosures can support a resonance if the right dimensional conditions exist. Higher frequencies will cause higher order harmonics to resonate even smaller cavities within a system enclosure. Thus, higher order harmonics are more likely to cause an emission testing failure than has been the case in the past with relatively slower speed processors.
Moreover, in many cases the resonance is so strong that no practical amount of box level shielding could reliably contain the radiated energy of higher speed systems to a level below the EMC radiated limits. Frequently, the mechanical dimensions for newly created cavities within a system enclosure are so well tuned to support an undesired resonance and resulting radiation, that reducing the level of source excitation via decoupling and filtering are ineffective. In this case, typical box shielding and source suppression techniques are generally found to be neither practical nor effective for higher frequency systems. Thus there is a need for an improved method and apparatus for effectively limiting EMC radiation which has resulted from the evolution and implementation of higher speed computer systems and other electronic devices.